One supporter of the Morgan family is a former Met witness protection officer and police whistleblower, originally from Wales, who knows Mr Morgan's mother, Isobel Hulsmann.

He uses the alias Frank Matthews on Twitter to maintain his anonymity but is known to the BBC and the Met.

He told BBC Radio Wales' current affair programme Eye on Wales the "Met have not been open and honest with this case".

"They hide behind the fact that they need to protect some of their information," he said.

"I disagree with that. This is a murder investigation, a clear case that could have been unravelled at the early stages.

"Until the Met comes clean we are left with no option than to believe that this is corruption.

"The next commissioner will have to think long and hard and if the commissioner comes up with the strategy, I'm sure it will pay dividends in the long run."

Image caption
Alastair Morgan has spoken out over his brother's death

Mr Morgan's brother Alastair remembers his first visit to the pub car park where his brother was murdered on 10 March 1987.

He said: "There was no crime scene cordon, nothing. I remember looking about and thinking he was brought here to kill him. This place was chosen to kill him."

Four men were charged with Mr Morgan's murder in 2008 but after 18 months of legal argument the prosecution's case collapsed in March 2011.

As home secretary in 2013, Ms May set up an independent, judge-led panel of experts to examine claims police corruption prevented a conviction.

A spokesman for the panel said: "The panel has started writing its report. However, as further new material requiring examination continued to be submitted throughout 2016, no publication date has yet been set.

"The aim is to get the report published before the Parliamentary summer recess."

Ms Dick was named as the Met's new commissioner last month, becoming the first woman to take charge of London's police force in its 188-year history.

Assistant Commissioner Martin Hewitt said: "The re-investigation into Daniel Morgan's murder identified, ever more clearly, how the initial inquiry failed the family and wider public.

"The MPS [Metropolitan Police Service] has stated that it is quite apparent that police corruption was a debilitating factor in that investigation, which is wholly unacceptable.

"The work of the Daniel Morgan Independent Panel is ongoing, and the MPS continues to work with them, as they seek to fully examine the circumstances of Daniel's murder, and the handling of the case since 1987."